Using the species Calliphora erythrocephala (a meroistic dipteran insect), a system has been developed which permits analysis of the patterns of gene activity during the production of a "mosaic" egg. Using this system, cloned DNA fragments transcribed during oogenesis will therefore be identified and further analysed with a view to establishing the following features concerning their expression. i) Are any of these genes transcribed exclusively during oogenesis? ii) Do transcripts for any of these genes show specific localization within the oocyte/egg? iii) Are incompletely processed transcripts from any of these genes stored within the egg? iv) Are transcripts from any of these genes activated at maturation/fertilization? Identification of the oocyte/egg proteins encoded by genes showing any of these characteristics would be attempted. These studies should yield valuable insights into the molecular mechanisms of embryological determination in a "mosaic" developmental system. An additional advantage comes from the choice of organism. C. erythrocephala is closely related to Drosophila melanogaster and thus any cloned genes identified as involved in determinative events in C. erythrocephala would be used to try and select ther homologues from D. melanogaster. If this proved possible, molecular genetical studies of their function could be performed.